1. Field of the Invention
The present invention is related to integrated circuit characterization methods and circuits, and more particularly to a circuit and method for measuring current in individual circuits and devices in a test integrated circuit.
2. Description of Related Art
Characterization integrated circuits are in common use to evaluate designs on processes and have become increasingly necessary as processes have shrunk. Ring oscillator circuits are frequently used in such designs to study device characteristic variations that lead to variations in circuit parameters such as delay. Performance screen ring oscillators (PSROs) are implemented along with functional circuits in a production integrated circuit to assist in production screening and as a design/process improvement tool. Since it is relatively easy to accurately measure the output frequency of a ring oscillator, and the output frequency is typically directly related to propagation delay, frequency measurements provide a effective means for studying process and design effects on circuit propagation delay.
A large number of ring oscillators may be implemented on a characterization die and individually enabled to provide local measurements of process characteristics. Further, multiple ring oscillators may also be used to study different designs and devices, such as transistors with varying channel width and the effect of such width variation on a circuit incorporating the transistor.
It is generally also desirable to know the current consumed by a particular circuit design/device and/or the variations of such currents across a die and across wafers, in particular since device strength variations are directly revealed. Measuring the operating current of a single ring oscillator that shares a power connection with other ring oscillators typically requires disabling all of the ring oscillators except for the ring oscillator under measurement. However, in such a scheme, the leakage current through all of the inactive oscillators can be significant and degrade the resolution of the current measurement. Further, the leakage current of a single ring oscillator cannot be measured, as the leakage due to a single inactive ring oscillator cannot be isolated in such a scheme. Alternative solutions using a separate power supply for each ring oscillator require an excess of pads and tester facility to perform multiple simultaneous ring oscillator frequency measurements and current measurements.
Similarly, it is desirable to characterize individual devices within a characterization integrated circuit. Existing characterization arrays typically use an array arrangement with rows and columns used to direct the current through an individual device, permitting voltage/current (IV) characterization. However, such circuits and test methodologies are not adequate for testing high speed circuits such as ring oscillators. The power delivery system of the above-described array circuits are not robust enough for proper operation, as high speed circuits typically require a power and ground distribution grid having a much lower impedance than the above-described array circuits. Therefore such array test circuits are typically only used for DC characterization. Further, such designs are not easily scalable, as the matrix must typically be increased by rows and columns, the drivers and sense circuits must be scaled to the new size, and the physical location of the row and column circuits must be adjusted as the matrix is expanded or contracted.
Therefore, it would be desirable to provide a method and circuit for accurately measuring ring oscillator operating and leakage current without requiring a separate power supply for each ring oscillator, and while providing for proper high speed operation of the ring oscillator circuit. It would further be desirable to provide a scalable method and circuit in which AC characteristics may be accurately measured by ring oscillator frequency and current measurement, and DC characteristics such as current-voltage curves of individual devices and AC characteristics of the device can be accurately measured. It would further be desirable to provide a method and circuit through which DC characteristics of individual devices can be correlated to AC operation of circuits such as ring oscillators.